The present invention is directed to a ferroelectric capacitor for use as a storage element in a dynamic random access memory and in particular, to a ferroelectric capacitor having asymmetric operation.
One of the major objectives in the fabrication of integrated circuit capacitors, which are the principal storage elements in (volatile or non-volatile) dynamic random access memories (DRAMs), is to increase the charge storage capacity of the capacitor despite the reduction of the lateral dimensions of the capacitor, thereby to allow for the fabrication of higher density memory devices. Since capacitance is inversely proportional to separation between two capacitor plates, this capacitance increase was initially accomplished by reducing the thickness of the dielectric (which was typically silicon dioxide or silicon nitride) separating the two plates. However, these dielectrics have a low dielectric constant .epsilon. and are limited in how thin they can be fabricated.
As a result, compounds or compositions with high dielectric constants were investigated, capacitance being related (directly) to the dielectric constant of the material (if any) between the plates. Recently, ferroelectric materials have been proposed for use as the dielectric in these capacitors due to the high dielectric constant of many of the ferroelectric materials. Such a capacitor is disclosed in Ramtron Corporation's U.S. Pat. No. 5,005,102 (Larson '102) by William Larson, entitled "Multilayer Electrodes For Integrated Circuit Capacitors," the disclosure of which is incorporated herein by reference. The capacitor described in Larson '102 utilizes a ferroelectric material as the dielectric between the top and bottom electrodes. The top and bottom electrodes have plate layers which are comprised of the same material. The result is a symmetrical ferroelectric capacitor.
An alternative approach is disclosed in U.S. Ser. No. 07/491,180 filed Mar. 9, 1990 entitled "Charge Magnified DRAM Cell" in the name of S. Sheffield Eaton. Jr., now U.S. Pat. No. 5,109,357. That approach is different because among other things, it uses relaxation properties of a ferroelectric material positioned as the capacitor dielectric, so that repeated reading/writing of the device gives a unique benefit beyond what is ordinarily gained.
The object of the present invention is to provide an asymmetric ferroelectric capacitor which has a greater charge storage capacity than the prior, symmetrical devices, but without having to reduce the thickness of the dielectric.